Quick Answer:
If you hear a random banging in your plumbing after a backflow preventer was installed, it’s usually caused by pressure changes or a valve reacting to brief surges — even when no fixtures seem to be running. Note the time of each bang and where it sounds loudest. If the pattern points to supply-side issues, a plumber can confirm safely and check pressure control components.
Why This Happens
Installing a backflow preventer can change how pressure and flow behave at the house connection. Common causes of intermittent bangs include:
- Water hammer from a valve snapping shut or opening quickly.
- Pressure spikes on the supply side from the city main or neighboring irrigation systems.
- Automatic fills or equipment (softeners, tanks) cycling briefly and triggering a check valve.
- Thermal expansion or relief cycling at the water heater — see Noise when water heater idle for a related situation.
Step-by-Step What to Do
1. Note the time of each bang
- Keep a simple log with the exact time, how long the sound lasts, and whether it’s repeated or random.
- Record day vs. night and any household activities at the time (laundry, sprinklers, etc.).
2. Check if it aligns with neighbors’ usage or city pressure changes
- Ask a nearby neighbor if they hear the same noise or if they run sprinklers at that time.
- Contact your water utility or check local notices for planned pressure changes or main work.
- If the banging lines up with supply-side events, that points away from a single fixture issue — see notes about Banging only during pressure surges.
3. Listen near the water heater and main entry to localize the surge
- Go to the water heater and place your ear near pipes (don’t touch hot surfaces). Also listen at the main shutoff and where the backflow device sits.
- Walk the system from the meter toward the house to see where the noise becomes louder. That helps determine whether it’s inside your piping or at the supply connection.
4. Confirm no fixtures or automatic devices are running
- Check toilets, irrigation controllers, water softeners, icemakers and any automatic valves. Turn off one system at a time to isolate the cause.
- If no device is cycling and the banging keeps happening, it suggests a supply-side pressure or valve reaction.
5. Record and wait before attempting repairs
- Keep the time log for at least a few days to show a plumber. Note any trends or exact times when the noise occurs.
- A professional can test pressure and identify whether a pressure reducing valve, backflow assembly or meter is causing the issue.
What Not to Do
- Don’t start opening walls or disabling safety devices.
- Don’t remove or tamper with the backflow preventer or pressure relief valves yourself.
- If bangs continue with no fixtures running, have a plumber evaluate pressure control components.
- Don’t make adjustments to municipal or service-side equipment; coordinate with your utility or a licensed plumber instead.
When to Call a Professional
Call a licensed plumber if any of the following apply:
- The banging continues after you’ve confirmed no household fixtures or automatic devices are running.
- You notice visible pipe movement, repeated loud bangs, or new leaks forming.
- Your recorded log shows the bangs line up with pressure events and you want the supply-side checked or a pressure-reducing valve tested.
A plumber can safely measure house pressure, inspect the backflow device, check the PRV or expansion tank, and contact the water utility if the issue appears to originate outside your property.
Safety Notes
- Turn off the main water shutoff only if you need to stop a leak or the plumber asks you to do so.
- Do not disable pressure-relief valves, expansion tanks or other safety devices; they protect your plumbing from dangerous pressure.
- If you see a leak or a fitting separating, shut off the main and call a professional immediately.
Common Homeowner Questions
- Could the backflow preventer be the cause? Yes — it can change flow behavior and let check valves react differently; a plumber can test it.
- Is this likely to damage my pipes if it happens occasionally? Occasional bangs are usually not catastrophic, but repeated pressure surges can stress fittings and should be checked.
- Can I fix it myself? Not safely in most cases; diagnosing supply-side pressure or adjusting safety devices is best left to a licensed plumber.
For more related articles, see the Random Banging With No Water Running hub.